This invention related to an interface for a supervised multi-input audible warning system.
Audible warning systems using piezoelectric devices for generating an alarm sound are well known. The piezo can be driven by a 555 timer configured as an oscillator. When an alarm condition is sensed, an input voltage is applied to the timer to drive the piezo. The vibration of the piezo generates the alarm sound.
The tone of the piezo is dictated by the frequency of the signal seen by the piezo, which is a function of the input voltage frequency to the timer. The frequency of the input voltage is controlled by the response characteristic of the passive elements comprising the network at the input of the timer. In essence, the tone of the piezo can be controlled by varying the equivalent capacitance seen by the input of the timer. This characteristic makes it possible to use one piezoelectric device to generate multiple tones merely by varying the capacitors used in the input circuitry. By placing multiple capacitors with different capacitances in parallel, the tone will vary depending on which of the capacitors has voltage applied across it. Furthermore, when the value of the capacitances vary significantly, if two or more capacitors have the same voltage applied across them, the capacitor with the higher or highest capacitance will drive the input frequency since that capacitor will substantially determine the equivalent capacitance seen by the input to the timer. This feature enables the differing alarm tones of one piezoelectric device to be prioritized.
By using a number of capacitors in parallel, one piezoelectric device can be used with more than one input thereby enabling a multitude of alarming conditions to be directed to a common circuit for generating the alarm sound. The ability of one piezo to generate a variety of tones depending on its input frequency (controllable by the capacitors chosen) permits one device to generate a different tone for each input. Such a design eliminates the number of piezoelectric devices needed for a given application.
Another desirable feature of an audible warning system is the ability to monitor or supervise the alarm-sensing and input circuitry for electrical faults. Without this feature an alarming condition may go unnoticed. The need for supervision is apparent considering that the bulk of the input circuit may be located in a control panel physically separated from the common circuitry located in an alarm unit. The distance between the control panel and the alarm unit increases the chances of an electrical fault in the wires connecting the two. Supervision alerts the system or an operator that one of the alarms may presently be disabled.
Supervising the circuitry typically consists of monitoring with a comparator a test current flowing through the input circuitry. Any fault in the circuit will be sensed by the comparator which will then alert the system and/or an operator of the problem. The test current often flows in the opposite direction of the input current to the timer. As a result, a diode placed between the input circuitry and the timer will prevent the test current from driving the timer.
Unfortunately, the two above-described desirable features, common circuitry from multiple inputs and supervised input circuitry, have been incompatible. In order for the two features to complement one another, the multiple input circuits must be isolated from each other. Without isolating either all current flow paths from the input circuitry to the common circuit or from the common circuit to the input circuitry, the supervising test current from one input could flow through the common circuitry and the input circuitry of another input resulting in an undesired actuation of the alarm. Therefore, the use of the diode, although adequate in single input warning systems, is inadequate in multiple input systems since it can only isolate one current path. Furthermore, without complete isolation of inputs, the disablement of one input could affect the operation of the other inputs by providing an unwanted current flow path.
Accordingly, it is an objective of the present invention to overcome the problems inherent in a supervised multi-input alarm utilizing common circuitry.
Another objective of the present invention is to combine the use of a supervised multi-input alarm with an addressable micro controller as part of the common circuit, thereby permitting selection from various programmed sounds depending on which of the supervised inputs is energized.